Fabrication, evaluation methodologies and models of slow-release fertilizers: A review

The development of slow-release fertilizers (SRFs) has attracted increasing attention, especially in the last two decades since the well-recognized environmental problem caused by overusing fertilizers. Different kinds of SRFs have been developed, including chemical or physical reactions, loading into the holders with porous and layer structures, coating, and holding by hydrogels, etc. Various techniques have been reviewed at different development stages and from different angles. Meanwhile, various methodologies have been developed and used to evaluate the release performances of different kinds of SRFs. However, the reported results are significantly different from the different evaluation methodologies. In this work, we have firstly briefly reviewed the development of SRFs according to the fabrication classification: (1) chemical synthesizing, (2) loading, (3) coating, (4) application of biodegradable polymers, and 5) hydrogel-based systems, then focused on reviewing different evaluation methodologies and their mechanisms. Various models used to describe and predicate release behaviors were also discussed. Generally, there are four kinds of evaluation methodologies: (1) immersing in water, (2) leaching by soil, (3) planting in greenhouse, and (4) yield comparison. It was found that most of the traditional synthesized products and coated fertilizers by conventional polymers are reasonably stable in water, so the method of immersing in water is suitable. Still, most of the water-sensitive coating or mixture-based products need to be evaluated through leaching by soil. Both greenhouse and yield comparison are simple and reliable methods and has been widely used for final evaluation and confirmation, but the weakness of time issue and unpredictable weather conditions limit their application during the initial developing step. It was found that there is no direct relationship between different methodologies since their releasing mechanisms are different even though the releasing trend is similar. Various evaluation methodologies need to be modified according to different products and adjusted according to laboratory capability (measurement techniques). The methods of element detection and environmental issues were also discussed. Various models used to describe and predicate releasing behaviors were reviewed in this paper. This paper also summarized the standards of SRFs from different countries. It was pointed out that the nutrient delivery to plants by actual controlled-release fertilizers should closely match the growing demand of the plants. A sustained supply of nutrients for the entire crop growth in a single dose (fertilizing) can save labor and handling costs. Furthermore, it is understandable that different plants grow favorably in different soil and conditions, so the fertilizers should also be evaluated in different environments and situations. The evaluation method should closely match plant growth requirements, such as rice in the water while corn is in solid soil.